The present invention relates to covered stents and to methods of making covered stents.
Balloon dilatation of blocked blood vessels;, mainly arteries, is one of the most common interventions in medical practice today. Thus, every year one million such interventions are made only in the coronary arteries of the heart. In the peripheral arteries, the numbers are similar. In about 60% of the cases, a stent of petal grid is inserted to support the vessel wall after dilatation Many of these interventions with a balloon or other instruments in the arteries create injuries and/or perforations of the vessel wall causing bleeding. In other cases, the vessel has undergone a spontaneous rupture. In these cases of vessel lesions and perforations a cover on the metal stents may close the hole or cover the damage of the inner layer of the vessel, called intima.
In addition to this, one out of one thousand newborn children are born with serious malformations or defects of the heart or the great vessels that require heart surgery with or without the use of a heart and lung machine. Many of these operations have the purpose to create new anatomic connections between blood vessels or between blood vessels and the atria or chambers of the heart.
Some products of covered stents (or stented grafts) that exist today have a cover of extruded fluoropolymers, e.g. polytetrafluoroethylene (ePTFE), either as a sheet wrapped around the stent, placed between a double sandwich stent, or the polymers are moulded to the stent. All these methods have disadvantages. The double stent (sandwich) is stiff and thick and will not easily bend around the corners and the bends that are common in blood vessels. The moulded stent on the other hand are prone to get cracks, deliminations and leakages.
U.S. Pat. No. 5,122,154 (Rhodes) discloses an endovascular graft comprising an elongated tubular sleeve having a plurality of expandable, ring-like, stent sections located at equidistantly spaced positions along the sleeve. The sleeve is formed of a thin and highly flexible material, such as expanded polytetrafluoroethylene, and is pleated; i.e. includes a plurality of longitudinally extending pleats. The pleated tube is normally in a compacted state, that is each of the pleats overlies and abuts a contiguous portion of an immediately adjacent pleat. The tube is arranged to be expanded to an expanded state wherein its plates open up and form a generally continuous curved surface.
U.S. Pat. No. 5,824,046 (Smith, et al.) discloses a composite intraluminar device and a method of forming such a device. This method comprises providing an elongate radially expandable tubular stent. A stent cover is formed from a longitudinal segment of unsintered ePTFE having a first longitudinal expanse and a first transverse expanse. The segment is expanded along the transverse expanse to provide a second transverse expanse greater than the first transverse expanse and a second longitudinal expanse less than the first longitudinal expanse. Finally, the expanded segment is wrapped exteriorly about the stent, with the second transverse expanse extending longitudinally along the elongate stent. In order to fixate the segment to the stent, the opposed longitudinal ends of the segment are overlapped and secured together, e.g. by an adhesive.
U.S. Pat. No. 5,922,393 (Jayraman) discloses a coated stent, wherein the stent is made from a flat sheet, the ends of which are assembled together by surface fusing. A coating is attached to the stent only at a single area of line contact on the outer surface of the stent with the remainder of the coating being larger than the unexpanded stent. Alternatively, a xe2x80x9cself-expandingxe2x80x9d stent is placed in a container of coating material and is coated. Thereafter, the stent is programmed in a desired manner and is subsequently physically compressed and kept inside a sheath.
U.S. Pat. No. 5,925,075 (Myers, et al) discloses a tubular intraluminal graft in the form of a tubular diametrically adjustable stent having a tubular covering of porous expanded polytetrafluoroethylene which is less than 0.10 mm thick. This covering may be on the exterior surface of the stent, or on the interior surface of the stent, or both. The covering may be affixed to the stent by an adhesive, which is preferably fluorinated ethylene propylene. The covering is affixed to the stent in its expanded state and the stent is then compressed mechanically to reduce the diameter of the covered stent. The resulting collapsed stent may then be expanded, e.g. by being heated.
U.S. Pat. No. 5,928,279 discloses further stented, radially expandable, tubular PTFE grafts manufactured by the individual components of the stented grafts being preassembled on a mandrel and subsequently heated to facilitate attachment of the PTFE layer(s) on one another and/or to the stent.
An object of the present invention is to provide a simplified method of making a covered stent and, more specifically, a flexible covered stent that will easily follow around the bends of blood vessels and also a stent that is not prone to get cracks, deliminations and leakages.
This object is attained by the method disclosed below and by the covered stent obtained thereby. Preferred alternatives of this method are also disclosed below.
Thus, a method of making a covered stent comprises the steps of producing a stent having a diameter corresponding to a substantially unexpanded state of the stent producing a tube of a film material which is shrinkable by exposure to an elevated temperature, the tube having a greater diameter than said stent; introducing the stent into the tube; and exposing the tube to the elevated temperature for reducing the diameter of the tube such that the stent is affixed in the tube.
Fluorinated ethylene-propylene (FEP) and polytetrafluoroethylene (PTFE) have the characteristic that they will shrink when they are exposed to high temperatures. Heating above 260xc2x0 C. for PTFE and above 200xc2x0 C. for FEP will make an extruded tube to shrink with a ratio of four to one. This means that a 4 mm in diameter PTFE tube will shrink to a diameter of 1 mm. The same tube may later again be dilated to its previous diameter of 4 mm under room or body temperatures, for instance during a balloon dilatation of a human blood vessel.
A special feature of the new covered stent is the option of a collar in one or both ends of the device for the purpose of anchoring of the device and for sealing purposes.
The covered stent may also be supplied with internal stents, preferably made of a memory metal (e.g. Nitinol) self-expanding material that will affix the PTFE cover. In this case, the cover is turned around the edge to the inside of the stent instead of creating a collar that goes outwards. This turning inside of the cover will strengthen its attachment to the stent and prevent a peel-off of the cover, this is especially important if the device is implanted into big vessels with a high pressure, like the aorta.
The presented method of covering the stents may be used on any alloy or polymer of stents and combinations of different types of alloy and polymer. This includes the use of any self-expanding metal of a memory metal like the Nitinol alloy, or different types of memory polymers.
Percutaneous insertion of the covered stent (by puncture and catheter technique) made according to the present invention may replace many pediatric heart operations and coronary artery bypass surgical procedures.